Resistive heater including wire resistor

ABSTRACT

A resistor heater includes an anode ( 10 ) arranged along one side and a cathode ( 20 ) arranged along the other side of a thin-line-shaped resistor ( 30 ). The anode ( 10 ) is connected to the resistor ( 30 ) at connections points (P 2 , P 3 ) by a plurality of branches ( 13, 14 ) arranged at a certain interval along the resistor ( 30 ). The cathode ( 20 ) is connected to the resistor ( 30 ) at connection points (P 1 , P 4 ) by branches ( 23, 24 ) arranged at a certain interval along the resistor ( 30 ). The connection points (P 1 , P 4 ) are located at positions shifted from one another along the resistor ( 30 ). A portion ( 31 ) of the resistor ( 30 ) located between the connections points (P 1 , P 2 ) and a portion ( 32 ) of the resistor ( 30 ) located between the connection points (P 3 , P 4 ) function as effective regions of the resistor ( 30 ).

TECHNICAL FIELD

The present invention relates to resistive heaters for electricallygenerating Joule heat and particularly relates to a resistive heaterincluding a wire resistor. The apparent (or superficial) electricalresistance of the resistive heater can be arbitrarily adjusted withoutchanging the shape of the wire resistor.

BACKGROUND ART

“Resistive heaters” for generating Joule heat by applying currents tothin-film resistors are widely used for various applications. Examplesof such resistive heaters include micro-sized resistive heaters placedon circuit substrates or semiconductors such as silicon. A large numberof attempts have been made to solve problems due to the size of themicro-sized resistive heaters. See, for example, Japanese UnexaminedPatent Application Publication Nos. 58-134764, 3-164270, and 61-219666and Japanese Patent No. 2811209. Techniques relating to these resistiveheaters are usually used to heat specific micro-regions (severalmicrometers square) or relatively large-area regions which are severalmillimeters to several centimeters square such that semiconductordevices mounted on such regions are heated.

In a case where a square region or a rectangular region which has asmall aspect ratio and which is therefore close to a square shape isheated, the shape of a resistive heater placed in the region is notparticularly limited. Therefore, a desired object can be readilyachieved by allowing the resistive heater to have such a shape that thetemperature distribution in the region can be desirably adjusted. Forthe electrical resistance of resistive heaters, since a large number ofholes can be bored in a sheet-shaped resistive heater, the electricalresistance of the heater can be readily adjusted by varying the sizeand/or number of the holes as disclosed in Japanese Unexamined PatentApplication Publication No. 58-134764.

For the resistive heater for heating the square or rectangular region,the temperature distribution obtained by the resistive heater and theelectrical resistance of the resistive heater can be adjusted by varyingthe shape of the resistive heater.

The electrical resistance of the resistive heater is a key factor todetermine the necessary performance, for example, the maximum voltage,of an external circuit for driving the resistive heater. If theresistive heater has a large electrical resistance, an extremely highvoltage must be applied to the driving circuit. In consideration of thevoltage (about 5 to 12 V) of a power supply, connected to a controlcircuit (usually including semiconductor devices), for controlling thetemperature, there is a problem in that these circuits cannot beconnected to a common power supply. Thus, it is necessary to adjust theelectrical resistance of the resistive heater.

On the other hand, an optical component, for example, “a thermoopticphase shifter”, used for optical communication includes a resistiveheater (see, for example, Japanese Unexamined Patent ApplicationPublication No. 6-34926). This resistive heater includes a resistorhaving a width of several micrometers to several tens micrometers and alength of about 2 to 5 mm. The length of the resistor is extremelygreater than the width thereof. Therefore, this resistive heater isdifferent from that resistive heater in that the resistor has a narrowline shape (a narrow stripe shape). The thermooptic phase shifterincludes an optical waveguide section having a width of about 5 μm and alength of about 2 to 5 mm. In order to selectively heat the opticalwaveguide section having such a shape using this resistive heater, theresistor must also have a narrow line shape.

Since the resistor has a width of several micrometers, it is difficultto arbitrarily adjust the electrical resistance of the resistor byvarying the shape thereof in the same manner as that described above.This is because a micromachining technique is necessary to shape theresistor.

The resistor is allowed to have a thickness of up to several hundredsnanometers because of the reason due to a semiconductor process used toform the thermooptic phase shifter. That is, the thickness of theresistor is limited. The number of materials for forming the opticalwaveguide section is not very large because such materials must havegood machinability, high stability, and high adhesion to a glassmaterial for forming the optical waveguide section.

As described above, in the resistive heater included in the opticalcomponent, there is a limitation that the resistor must have a narrowline shape; hence, it is very difficult to prepare a heating element (inparticular, a heating element with low electrical resistance) withdesired electrical resistance properties by improving the shape of theresistor. Furthermore, it is not easy to adjust the thickness of theresistive heater or change a material for forming the resistive heateras required because of process and material constraints.

There are known techniques relating to the present invention asdescribed below.

Japanese Unexamined Patent Application Publication No. 2001-301219discloses a thermal print head including a wire resistor. The thermalprint head, as specified in claim 1 of this patent document, includes “alinear resistor, a power supply line, a grounding line, and anintegrated circuit device, wherein the integrated circuit deviceincludes a plurality of transistors each including respective firstelectrodes connected to the power supply and respective secondelectrodes connected to the grounding line and also includes a pluralityof pads for connecting the second electrodes to the grounding line andthe resistor generates heat when a current is applied to the resistor bybring the transistors into conduction”. According to such aconfiguration, the following advantages can be achieved: “the secondelectrodes can be connected to the pads with short wires, the wirestherefore have low resistance, a difference in wiring resistance betweenthe transistors is small, electricity consumption is low, the life of abattery included in the thermal print head is long if the thermal printhead is of a portable type, the thermal print head can be driven with alow-voltage battery because a voltage drop due to the wiring resistanceis small, the quality of an image formed by the thermal print head ishigh because a difference in wiring resistance between the transistorsis small and because a difference in temperature between portions of theresistor is small”.

In the thermal print head disclosed in Japanese Unexamined PatentApplication Publication No. 2001-301219, the resistor and the powersupply line are connected to each other with a plurality of spaced wiresand the first electrodes of the transistors are connected to theresistor with wires. The first and second electrodes of the transistorscorrespond to the drains and sources of MOS transistors, respectively.If one of the transistors in the integrated circuit device is turned on,a current flows from the power supply line to the grounding line throughthe resistor and the transistor. Since the current flows in two wiresfor connecting the power supply line to the resistor and flows in aportion of the resistor that is sandwiched between the two wires, theresistor portion can be selectively heated.

Japanese Unexamined Patent Application Publication No. 2002-008901discloses a thin-film resistor, a hybrid IC, and a microwave monolithicintegrated circuit (MMIC). In the thin-film resistor, “a first electrodeand second electrode connected to thin-film resistor portions havenarrow, irregular sections extending in the direction that the first andsecond electrodes face to each other; sides of the irregular sections ofthe first and second electrodes are arranged at predetermined intervals;and the thin-film resistor portions are arranged between the sidesfacing to each other”. That is, in the thin-film resistor, an endsection of the first electrode is shaped so as to have an interdigitalshape so that the irregular electrode sections are formed, an endsection of the second electrode is shaped so as to have an interdigitalshape so that the irregular electrode sections are formed, and theelectrode sections are engaged with each other in such a manner that theinterdigital electrode sections of the second electrode are placed inspaces between the interdigital electrode sections of the firstelectrode. The thin-film resistor portions are separately placed inspaces between the interdigital electrode sections engaged with eachother.

According to such a configuration, the following advantages can beachieved: “the thin-film resistor can be shaped so as to have a sizeclose to the width of wires and a region for forming the thin-filmresistor can therefore be formed so as to have a desired characteristicimpedance”.

If the operational stability and reliability of resistive heaters areregarded as most important, tantalum nitride (TaN) is usually used toprepare resistors. Thin-film heaters, made of TaN, for semiconductorcircuits have a large electrical resistance because the resistivity ofTaN is usually high, 200 to 300 μΩ·cm, under conditions for stablyforming layers. If, for example, a TaN layer is processed into finewires having a thickness of 200 nm a width of 10 μm, and a length of 2mm, the wires have an electrical resistance of 2 to 3 kΩ. In order toallow a wire resistor, made of TaN, having such an electrical resistanceto generate 300 mW of heat, the voltage necessary to energize the wireresistor is very high, 17 to 30 V.

An attempt to prepare a small-sized, precisely controllable heatingelement including a TaN wire resistor causes a problem, i.e., anincrease in the size of a driving power supply. Hence, the attempt isimpossible. This can be applied to titanium nitride (TiN), as well asTaN, having a relatively large resistivity.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a resistive heaterincluding a wire resistor and a thermooptic phase shifter including sucha resistive heater. If the wire resistor is made of a material, such astantalum nitride or titanium nitride, having a relatively largeresistivity, the apparent electrical resistance of the resistive heater(the superficial electrical resistance of the resistive heater) is lessthan the electrical resistance estimated from the material.

It is another object of the present invention to provide a resistiveheater of which the apparent electrical resistance can be arbitrarilyadjusted and which includes a wire resistor and a thermooptic phaseshifter including such a resistive heater.

It is another object of the present invention to provide a resistiveheater, including a wire resistor, for producing heat of which theamount can be controlled with a simple electronic circuit and athermooptic phase shifter including such a resistive heater.

In the thermal print head disclosed in Japanese Unexamined PatentApplication Publication No. 2001-301219, a current flows through theresistor portion sandwiched between the two wires for connecting thepower supply line to the resistor having a narrow line shape, wherebythe resistor portion is selectively allowed to generate heat. However,such a technique is not useful in achieving the following object of thepresent invention: “if the narrow resistor is made of a material, suchas tantalum nitride or titanium nitride, having a relatively largeresistivity, the apparent electrical resistance of the resistive heateris less than the electrical resistance estimated from the material”. Thethermal print head is quite different from a resistive heater accordingto the present invention.

The thin-film resistor disclosed in Japanese Unexamined PatentApplication Publication No. 2002-008901 does not have a wire shape andis therefore different from “a resistive heater including a wireresistor” as specified herein. An object (purpose) thereof is asfollows: “when the first and second electrodes have a size significantlygreater than the width of lines, the first and second electrodes have acharacteristic impedance (for example, 50Ω) unsuitable for transmissionlines; hence, desired operations cannot be performed due tomiss-matching”. An effect thereof is as follows: “the resistive heatercan be formed so as to have a size close to the line width and a regionfor forming the thin-film resistor can therefore be formed so as to havea desired characteristic impedance”. As is clear from the object andeffect, the thin-film resistor is quite different from a resistiveheater of the present invention.

Further other objects of the present invention will become apparent fromdescriptions below and the accompanying drawings although the objectsare not described above.

(1) A resistive heater of the present invention includes:

a wire resistor;

a first electrode, placed on a side of the resistor, extending along theresistor; and

a second electrode, placed on the side opposite to the first electrode,extending along the resistor,

wherein the first electrode is connected to a plurality of first nodesplaced on the resistor with branches spaced along the resistor,

the second electrode is connected to a plurality of second nodes placedon the resistor with branches spaced along the resistor,

the second nodes are spaced from the first nodes in the longitudinaldirection of the resistor, and the resistor has effective regions eachsandwiched between one of the first nodes and one of the second nodesthat is adjacent to the first connection.

(2) In the resistive heater of the present invention, the firstelectrode is placed on a side of the resistor and extends along theresistor and the second electrode is placed on the side opposite to thefirst electrode and extends along the resistor. The first electrode isconnected to the first nodes placed on the resistor with the branchesspaced along the resistor and the second electrode is connected to thesecond nodes placed on the resistor with the branches spaced along theresistor. The second nodes are spaced from the first nodes in thelongitudinal direction of the resistor and the resistor has effectiveregions each sandwiched between one of the first nodes and one of thesecond nodes that is adjacent to the first connection.

(3) In a preferable example of the resistive heater of the presentinvention, the first and second nodes are alternately arranged in thelongitudinal direction of the resistor.

(4) A thermooptic phase shifter of the present invention includes:

an optical waveguide; and

the resistive heater, according to Items (1) to (3) described above, forheating the optical waveguide,

wherein the resistor included in the resistive heater extends along theoptical waveguide.

(5) The thermooptic phase shifter of the present invention includes theresistive heater, according to Items (1) to (3) described above, forheating the optical waveguide and the resistor of the resistive heaterextends along the optical waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view showing a resistive heater according to a firstexample of the present invention.

FIG. 1B is a diagram showing an equivalent circuit of the resistiveheater shown in FIG. 1A.

FIG. 2A is a plan view showing a resistive heater according to a secondexample of the present invention.

FIG. 2B is a diagram showing an equivalent circuit of the resistiveheater shown in FIG. 2A.

FIG. 3A is a plan view showing a resistive heater according to a thirdexample of the present invention.

FIG. 3B is a diagram showing an equivalent circuit of the resistiveheater shown in FIG. 3A.

FIG. 4A is a plan view showing a resistive heater according to a fourthexample of the present invention.

FIG. 4B is a diagram showing an equivalent circuit of the resistiveheater shown in FIG. 4A.

FIGS. 5A to 5E are sectional views showing principal parts of theresistive heater of the fourth example in the order of manufacturingsteps.

FIG. 6 is a plan view showing a configuration of a thermooptic phaseshifter according to a fifth example of the present invention.

FIG. 7A is a plan view showing a known resistive heater.

FIG. 7B is a diagram showing an equivalent circuit of the knownresistive heater shown in FIG. 7A.

BEST MODE FOR CARRYING OUT THE INVENTION

A resistive heater according to the present invention will now bedescribed.

According to the present invention, the wire resistor is divided into aplurality of effective regions and non-effective regions other than theeffective regions with the first and second nodes. If the resistor ismade of a material, such as tantalum nitride or titanium nitride, havinga relatively large resistivity, the apparent electrical resistance (thesuperficial electrical resistance of the resistive heater) is less thanthe electrical resistance of the material because the effective regionsare connected to the first and second electrodes in parallel. Therefore,the amount of heat generated by the resistive heater can be controlledwith a simple electronic circuit.

The number, position, and/or length of the effective regions can bevaried by changing the number and/or position of the branches of thefirst or second electrode. Therefore, the apparent electrical resistanceof the resistive heater (the superficial electrical resistance of theresistive heater) can be readily adjusted to any value.

In a preferable example of the resistive heater according to the presentinvention, the first electrode and the second electrode, i.e., apositive electrode and a negative electrode, are alternately arranged inthe longitudinal direction of the resistor; hence, heat is generatedfrom substantially the whole of the resistor. Therefore, there is anadvantage in that the temperature of the resistive heater is uniform inthe longitudinal direction thereof. Furthermore, since the temperatureof the resistive heater is uniform in the longitudinal direction, theresistor can be prevented from being deteriorated. Therefore, there isan advantage in that the resistive heater has high long-termreliability.

In another preferable example of the resistive heater according to thepresent invention, the first and second nodes are arranged such that theeffective regions extending in the longitudinal direction of theresistor have the same length. In this example, the effective regionsgenerate the same amount of heat (calorific power); hence, thetemperature of the resistive heater is uniform in the longitudinaldirection. Therefore, there is an advantage in that a stress applied tothe resistor can be greatly reduced. Furthermore, the effective regionssandwiched between the first and second electrodes (for example, apositive electrode and a negative electrode) have the same electricalresistance; hence, there is an advantage in that the resistive heatercan be readily designed, controlled, and operated.

In another preferable example of the resistive heater according to thepresent invention, any two of the first and second nodes are each placedat one of both ends of the resistor. In this example, since the whole ofthe resistor can be effectively used, the apparent electrical resistanceR′ of the resistive heater is determined depending only on the number nof the effective regions of the resistive heater. This means that theapparent electrical resistance R′ can be designed using only the numbern of the effective regions; hence, there is an advantage in that theresistive heater can be readily designed.

In another preferable example of the resistive heater according to thepresent invention, the resistor is made of a material principallycontaining tantalum nitride or titanium nitride. In this example, thematerial for forming the resistor has high reliability and the tantalumnitride and titanium nitride have relatively high resistivity; hence,there is an advantage in that advantages of the present invention can bemaximized.

In another preferable example of the resistive heater according to thepresent invention, the first and second electrodes are made of amaterial containing at least two selected from the group consisting ofgold, platinum, chromium, titanium, copper, aluminum, titanium nitride,and tantalum nitride. In this example, the first and second electrodeshave an electrical resistance sufficiently less than that of theresistor; hence, there is an advantage in that the resistor canefficiently generate heat.

Thus, the same advantage as that of the resistor of the presentinvention can be achieved. Furthermore, there is an advantage in that itis preferable to efficiently use heat generated by the resistive heaterof the present invention.

Examples of the resistor according to the present invention will now bedescribed in detail with reference to the accompanying drawings.

FIRST EXAMPLE

FIG. 1A is a plan view showing a configuration of a resistive heateraccording to a first example of the present invention and FIG. 1B is adiagram showing an equivalent circuit of the resistive heater.

With reference to FIG. 1A, the resistive heater of the first example isplaced on an insulating substrate (not shown) and includes a wireresistor 30 having a predetermined length; a positive electrode 10,placed on a side (the upper side in FIG. 1A) of the resistor 30,extending along the resistor 30; and a negative electrode 20, placed onthe side (the lower side in FIG. 1A) opposite to the positive electrode10, extending along the resistor 30.

The resistor 30 extends straight on the substrate and has a uniformwidth of, for example, 10 μm. The resistor 30 has a length of, forexample, 2 mm and a thickness of, for example, 200 nm. The resistor 30is made of TaN or TiN.

The positive electrode 10 extends along the resistor 30 and they arespaced from each other. The negative electrode 20 extends along theresistor 30 and they are spaced from each other. The positive electrode10 and the negative electrode 20 are parallel to the resistor 30. Thepositive electrode 10 and the negative electrode 20 each include aconductive body having a resistivity sufficiently less than that of theresistor 30. The conductive body has a triple layer structure consistingof an aluminum (Al) layer, a titanium (Ti) layer, and a gold (Au) layer.

The positive electrode 10 includes a connection 11, an extension 12, andtwo branches 13 and 14. The connection 11 is connected to an externalcircuit, that is, the connection 11 is used as a bonding pad. Theextension 12 has a stripe shape and extends from the connection 11 inparallel to the resistor 30. The branches 13 and 14 are arranged betweenthe extension 12 and the resistor 30. The branches 13 and 14 have astripe shape, are narrower than the extension 12, meet the resistor 30and the extension 12 at right angles, and are spaced along the resistor30. The branch 13 is connected to a node P2 placed on the resistor 30.The branch 14 is connected to a node P3 placed on the resistor 30. Thepositive electrode 10 has an electrical resistance sufficiently lessthan that of the resistor 30.

The negative electrode 20 as well as the positive electrode 10 includesa connection 21, an extension 22, and two branches 23 and 24. Theconnection 21 is connected to an external circuit, that is, theconnection 21 is used as a bonding pad. The extension 22 has a stripeshape and extends from the connection 21 in parallel to the resistor 30.The branches 23 and 24 are arranged between the extension 22 and theresistor 30. The branches 23 and 24 have a stripe shape, are narrowerthan the extension 22, meet the resistor 30 and the extension 22 atright angles, and are spaced along the resistor 30. The branch 23 isconnected to a node P1 placed on the resistor 30. The branch 24 isconnected to a node P4 placed on the resistor 30. The negative electrode20 also has an electrical resistance sufficiently less than that of theresistor 30.

The node P2 connected to the branch 13 of the positive electrode 10 isspaced from the node P3 connected to the branch 14 of the positiveelectrode 10 in the longitudinal direction of the resistor 30. The nodeP1 connected to the branch 23 of the negative electrode 20 is spacedfrom the node P4 connected to the branch 24 of the negative electrode 20in the longitudinal direction of the resistor 30. The node P1 connectedto the negative electrode 20 is spaced from the nodes P2 and P3 placedon the positive electrode 10 in the longitudinal direction of theresistor 30. The node P4 placed on the negative electrode 20 is spacedfrom the nodes P2 and P3 placed on the positive electrode 10 in thelongitudinal direction of the resistor 30. That is, the nodes P1 to P4are located at different positions.

In the resistive heater according to the first example of the presentinvention, since the nodes P1 to P4 are arranged as described above, theresistor 30 has an effective region 31 sandwiched between the node P2placed on the positive electrode 10 and the node P1 placed on thenegative electrode 20 and an effective region 32 sandwiched between thenode P3 placed on the positive electrode 10 and the node P4 placed onthe negative electrode 20 (see FIGS. 1A and 1B). Regions other than theeffective regions 31 and 32 do not function as “resistive regions” andare therefore referred to as non-effective regions.

If a predetermined voltage is applied to the positive electrode 10 froma power supply and the negative electrode 20 is grounded, currents flowfrom the positive electrode 10 to the negative electrode 20. In thissituation, a current flows from the branch 13 of the positive electrode10 to the branch 23 of the negative electrode 20 through the effectiveregion 31 of the resistor 30 and a current flows from the branch 14 ofthe positive electrode 10 to the branch 24 of the negative electrode 20through the effective region 32 of the resistor 30. No current flowsbetween the branches 13 and 14 of the positive electrode 10. This isbecause the branches 13 and 14 have the same potential. As a matter ofcourse, no currents flow in both end regions of the resistor 30, thatis, one end region located on the left side of the branch 23 of thenegative electrode 20 and the other end region located on the right sideof the branch 24 of the negative electrode 20.

If the electrical resistances of the positive and negative electrodes 10and 20 are negligible, an equivalent circuit of the resistor 30 is asshown in FIG. 1B, wherein R1 represents the electrical resistance of theeffective region 31 and R2 represents the electrical resistance of theeffective region 32. The apparent electrical resistance of the resistiveheater (the superficial electrical resistance of the resistive heater)according to the first example of the present invention is equal to theelectrical resistance of a circuit including two resistors, connected toeach other in parallel, having an electrical resistance equal to R1 orR2. Therefore, the apparent electrical resistance of the resistiveheater is greatly less than the electrical resistance estimated from theresistivity of the resistor 30.

As described above, although the resistor 30 is made of a material, suchas TaN or TiN, having a relatively large resistivity, the resistiveheater (see FIGS. 1A and 1B) according to the first example of thepresent invention has an apparent electrical resistance less than thatestimated from the material. Therefore, the amount of heat generated bythe resistive heater can be controlled with a simple electronic circuit.

The number, position, and/or length of the effective regions of theresistor 30 can be varied by changing the number and/or position of thebranches of the positive or negative electrode 10 or 20. Therefore, theapparent electrical resistance of the resistive heater can be adjustedto any value.

Thus, a control circuit, a driving circuit, and other circuits can beconnected to a common power supply; hence, a small-sized, user-friendlyheating element can be achieved.

FIG. 7A is a plan view showing a configuration of a known resistiveheater, which is a comparative example, and FIG. 7B is a diagram showingan equivalent circuit of the known resistive heater. The known resistiveheater shown in FIG. 7A includes a wire resistor 130, a positiveelectrode 110 connected to one end of the resistor 130, a negativeelectrode 120 connected to the other end thereof. The equivalent circuitthereof is as shown in FIG. 7B and the apparent electrical resistance ofthis resistive heater is equal to the electrical resistance R of theresistor 130. Therefore, once the resistor 130 is formed, the apparentelectrical resistance of this resistive heater cannot be adjusted.

The configuration of the resistive heater according to the first exampleof the present invention can be generally described as below.

The following equation (1) holds:(1/R′)=(1/R)×{(1/m ₁)+(1/m ₂)+ . . . +(1/m _(n))}  (1)wherein R′ represents the apparent electrical resistance of theresistive heater observed from an external driving circuit, m₁, m₂, . .. , and m_(n) represent the percentages of effective regions in theresistive heater and are less than 1, and n represents the number of theeffective regions and is not equal to 0.

Since m₁ to m_(n) are less than 1, R′ is less than R. Accordingly, theresistive heater has an electrical resistance less than that of theknown resistive heater shown in FIGS. 7A and 7B.

SECOND EXAMPLE

FIG. 2A is a plan view showing a configuration of a resistive heateraccording to a second example of the present invention and FIG. 2B is adiagram showing an equivalent circuit of the resistive heater.

With reference to FIG. 2A, the resistive heater of the second example isplaced on an insulating substrate (not shown) and includes a wireresistor 30A having a predetermined length; a positive electrode 10A,placed on a side (the upper side in FIG. 2A) of the resistor 30A,extending along the resistor 30A; and a negative electrode 20A, placedon the side (the lower side in FIG. 2A) opposite to the positiveelectrode 10A, extending along the resistor 30A.

Other components of the resistor 30A are the same as those of theresistor 30 of the first example and the description of the resistor 30Ais therefore omitted.

The positive electrode 10A extends along the resistor 30A and they arespaced from each other. The negative electrode 20A extends along theresistor 30A and they are spaced from each other. The positive electrode10A and the negative electrode 20A are parallel to the resistor 30A. Thepositive electrode 10 and the negative electrode 20 each include aconductive body having a resistivity sufficiently less than that of theresistor 30A.

The positive electrode 10A includes a connection 11A, an extension 12A,and three branches 13A, 14A, and 15A. The connection 11A is connected toan external circuit. The extension 12A has a stripe shape and extendsfrom the connection 11A in parallel to the resistor 30A. The branches13A, 14A, and 15A are arranged between the extension 12A and theresistor 30A. The branches 13A, 14A, and 15A have a stripe shape, arenarrower than the extension 12A, meet the resistor 30A and the extension12A at right angles, and are spaced along the resistor 30.

The branch 13A is connected to a node P11 placed on the resistor 30A.The branch 14A is connected to a node P13 placed on the resistor 30A.The branch 15A is connected to a node P15 placed on the resistor 30A.The positive electrode 10A has an electrical resistance sufficientlyless than that of the resistor 30A.

The negative electrode 20A as well as the positive electrode 10Aincludes a connection 21A, an extension 22A, and three branches 23A,24A, and 25A. The connection 21A is connected to an external circuit.The extension 22A has a stripe shape and extends from the connection 21Ain parallel to the resistor 30A. The branches 23A, 24A, and 25A arearranged between the extension 22A and the resistor 30A. The branches23A, 24A, and 25A have a stripe shape, are narrower than the extension22A, meet the resistor 30A and the extension 22A at right angles, andare spaced along the resistor 30A.

The branch 23A is connected to a node P12 placed on the resistor 30A.The branch 24A is connected to a node P14 placed on the resistor 30A.The branch 25A is connected to a node P16 placed on the resistor 30A.The negative electrode 20A also has an electrical resistancesufficiently less than that of the resistor 30A.

The nodes P11, P13, and P15 connected to the branches 13A, 14A, and 15A,respectively, on the positive electrode 10A are spaced from each otherin the longitudinal direction of the resistor 30A. The nodes P12, P14,and P16 connected to the branches 23A, 24A, and 25A, respectively, onthe positive electrode 10A are spaced from each other in thelongitudinal direction of the resistor 30A. The node P12 on the negativeelectrode 20A is spaced from the nodes P11, P13, and P15 on the positiveelectrode 10A in the longitudinal direction of the resistor 30A. Thenode P14 on the negative electrode 20A is spaced from the nodes P11,P13, and P15 on the positive electrode 10A in the longitudinal directionof the resistor 30A. The node P16 on the negative electrode 20A isspaced from the nodes P11, P13, and P15 on the positive electrode 10A inthe longitudinal direction of the resistor 30A. That is, the nodes P11to P16 are located at different positions.

In the resistive heater according to the second example of the presentinvention, since the nodes P11 to P16 are arranged as described above,the resistor 30A has effective regions 31A, 32A, 33A, 34A, and 35A (seeFIGS. 2A and 2B). Regions other than the five effective regions 31A,32A, 33A, 34A, and 35A do not function as “resistive regions” and aretherefore referred to as non-effective regions. The region 31A is aportion of the resistor 30A that is sandwiched between the node P11 onthe positive electrode 10A and the node P12 on the negative electrode20A. The region 32A is a portion of the resistor 30A that is sandwichedbetween the node P13 on the positive electrode 10A and the node P12 onthe negative electrode 20A. The region 33A is a portion of the resistor30A that is sandwiched between the node P13 on the positive electrode10A and the node P 14 on the negative electrode 20A. The region 34A is aportion of the resistor 30A that is sandwiched between the node P15 onthe positive electrode 10A and the node P 14 on the negative electrode20A. The region 35A is a portion of the resistor 30A that is sandwichedbetween the node P15 on the positive electrode 10A and the node P 16 onthe negative electrode 20A.

If a predetermined voltage is applied to the positive electrode 10A froma power supply and the negative electrode 20A is grounded, currents flowfrom the positive electrode 10A to the negative electrode 20A. In thissituation, a current flows from the branch 13A of the positive electrode10A to the branch 23A of the negative electrode 20A through theeffective region 31A of the resistor 30A, a current flows from thebranch 14A of the positive electrode 10A to the branch 23A of thenegative electrode 20A through the effective region 32A of the resistor30A, a current flows from the branch 14A of the positive electrode 10Ato the branch 24A of the negative electrode 20A through the effectiveregion 33A of the resistor 30A, a current flows from the branch 15A ofthe positive electrode 10A to the branch 24A of the negative electrode20A through the effective region 34A of the resistor 30A, and a currentflows from the branch 15A of the positive electrode 10A to the branch25A of the negative electrode 20A through the effective region 35A ofthe resistor 30A. No currents flow in a portion located outside thebranch 13A (the node P 11) of the positive electrode 10A and a portionlocated outside the branch 25A (the node P 16) of the negative electrode20A.

If the electrical resistances of the positive and negative electrodes10A and 20A are negligible, an equivalent circuit of the resistor 30A isas shown in FIG. 2B, wherein R1, R2, R3, R4, and R5 represent theelectrical resistance of the effective regions 31A, 32A, 33A, 34A, and35A, respectively. The apparent electrical resistance R′ of theresistive heater according to the second example of the presentinvention is equal to the electrical resistance of a circuit includingfive resistors, connected to each other in parallel, having anelectrical resistance equal to R1, R2, R3, R4, or R5. Therefore, theapparent electrical resistance R′ of the resistive heater is greatlyless than the electrical resistance estimated from the resistivity ofthe resistor 30A.

The resistive heater of the second example has the same advantages asthose of the resistive heater of the first example. Furthermore, theapparent electrical resistance R′ of the resistive heater of the secondexample is less than that of the resistive heater of the first example.

In the resistive heater of the second example, since the nodes P11, P13,and P15 of the positive electrode 10A and the nodes P12, P14, and P16 ofthe negative electrode 20A are alternately arranged in the longitudinaldirection of the resistor 30A, each region of the resistor 30A that issandwiched between the nodes adjacent to each other effectivelyfunctions. Therefore, the resistor 30A has no non-effective regionsexcept for both end regions thereof. This means that available regionsof the resistor 30A can be fully used. Therefore, almost all regions ofthe resistor 30A are allowed to generate heat; hence, the temperaturethereof is uniform. Furthermore, since the resistive heater of thesecond example generates heat from the effective regions having a largerarea as compared to the resistive heater of the first example, a loadapplied to the resistor 30A is distributed; hence, there is an advantagein that this resistive heater can be prevented from being deteriorated.

THIRD EXAMPLE

FIG. 3A is a plan view showing a configuration of a resistive heateraccording to a third example of the present invention and FIG. 3B is adiagram showing an equivalent circuit of the resistive heater.

With reference to FIG. 3A, the resistive heater of the third example aswell as the resistive heater of the first example is placed on aninsulating substrate (not shown) and includes a wire resistor 30B havinga predetermined length; a positive electrode 10B, placed on a side (theupper side in FIG. 3A) of the resistor 30B, extending along the resistor30B; and a negative electrode 20B, placed on the side (the lower side inFIG. 3A) opposite to the positive electrode 10B, extending along theresistor 30B.

Other components of the resistor 30B are the same as those of theresistor 30 of the first example and the description of the resistor 30Bis therefore omitted.

The positive electrode 10B has the same configuration as that of thepositive electrode 10 of the first example except that the positiveelectrode 10B includes three branches 13B, 14B, and 15B. Referencenumeral 11B represents a connection and reference numeral 12B representsan extension.

The branch 13B of the positive electrode 10B is connected to a node P21placed on the resistor 30B. The branch 14B is connected to a node P23placed on the resistor 30B. The branch 15B is connected to a node P24placed on the resistor 30B.

The negative electrode 20B has the same configuration as that of thenegative electrode 20 of the first example. Reference numeral 21Brepresents a connection, reference numeral 22B represents an extension,and reference numerals 23B and 24B represent branches.

The branch 23B of the negative electrode 20B is connected to a node P22placed on the resistor 30B. The branch 24B is connected to a node P25placed on the resistor 30B.

In this example, an effective region 31B is present between the nodesP21 and P22 of the resistor 30B, an effective region 32B is presentbetween the nodes P22 and P23, and an effective region 33B is presentbetween the nodes P24 and P25. The nodes P21 to P25 are arranged suchthat the three effective regions 31B, 32B, and 33B have the same length.Therefore, the effective regions 31B, 32B, and 33B have the sameelectrical resistance.

If the electrical resistances of the positive and negative electrodes10B and 20B are negligible, an equivalent circuit of the resistor 30B isas shown in FIG. 3B, wherein R1, R2, and R3 (R1=R2=R3) represent theelectrical resistance of the effective regions 31A, 32A, and 33A,respectively. The apparent electrical resistance R′ of the resistiveheater according to the third example of the present invention is equalto the electrical resistance of a circuit including three resistors,connected to each other in parallel, having an electrical resistanceequal to R1, R2, or R3. Therefore, the apparent electrical resistance R′of the resistive heater is greatly less than the electrical resistanceestimated from the resistivity of the resistor 30B.

As described above, the resistive heater of the third example has thesame advantage as that of the resistive heater of the first example.

The following equation (2) holds:1/R′=(1/R)×(n/m)  (2)wherein R′ represents the apparent electrical resistance of theresistive heater observed from an external driving circuit, n representsthe number of the effective regions of the resistive heater and is notequal to 0, and m represents the percentage of the effective regions inthe resistive heater and is less than 1.

Therefore, there is an advantage in that the apparent electricalresistance R′ of the resistive heater can be readily determined usingthe number n of the effective regions of the resistive heater and thepercentage m of the effective regions in the resistive heater.

FOURTH EXAMPLE

FIG. 4A is a plan view showing a configuration of a resistive heateraccording to a fourth example of the present invention and FIG. 4B is adiagram showing an equivalent circuit of the resistive heater.

With reference to FIG. 4A, the resistive heater of the fourth example,as well as that of the first example, is placed on an insulatingsubstrate (not shown) and includes a wire resistor 30C having apredetermined length; a positive electrode 10C, placed on a side (theupper side in FIG. 4A) of the resistor 30C, extending along the resistor30C; and a negative electrode 20C, placed on the side (the lower side inFIG. 4A) opposite to the positive electrode 10C, extending along theresistor 30C.

The resistor 30C has no region extending out of a node P31 nor P36. Inother words, the resistor 30C has substantially the same configurationas that of the resistor 30A, shown in FIGS. 2A and 2B, according to thesecond example except that the nodes P31 and P36 are each placed at oneof both ends of the resistor 30C; hence, the description of the resistor30C is omitted.

The positive electrode 10C has the same configuration as that of thepositive electrode 10A of the second example except that the positionsof three branches 13C, 14C, and 15C included in the positive electrode10C are different from those of the branches of the positive electrode10A of the second example. Reference numeral 11C represents a connectionand reference numeral 12C represents an extension.

The branch 13C of the positive electrode 10C is connected to the nodeP31 of the resistor 30C. The branch 14C is connected to a node P33placed on the resistor 30C. The branch 15C is connected to a node P35placed on the resistor 30C.

The negative electrode 20C has the same configuration as that of thenegative electrode 20A of the second example except that the positionsof three branches 23C, 24C, and 25C included in the negative electrode20C are different from those of the branches of the negative electrode20A of the second example. Reference numeral 21C represents a connectionand reference numeral 22C represents an extension.

The branch 23C of the negative electrode 20C is connected to a node P32placed on the resistor 30C. The branch 24C is connected to a node P34placed on the resistor 30C. The branch 25C is connected to the node P36of the resistor 30C.

In this example, an effective region 31C is present between the nodesP31 and P32 of the resistor 30C, an effective region 32C is presentbetween the nodes P32 and P33, an effective region 33C is presentbetween the nodes P33 and P34, an effective region 34C is presentbetween the nodes P34 and P35, and an effective region 35C is presentbetween the nodes P35 and P36. The nodes P31 to P36 are arranged suchthat the five effective regions 31C, 32C, 33C, 34C, and 35C have thesame length. Therefore, the effective regions 31C, 32C, 33C, 34C, and35C have the same electrical resistance.

If the electrical resistances of the positive and negative electrodes10C and 20C are negligible, an equivalent circuit of the resistor 30C isas shown in FIG. 4B, wherein R1, R2, R3, R4, and R5 (R1=R2=R3=R4=R5)represent the electrical resistance of the effective regions 31A, 32A,33A, 34A, and 35A, respectively. The apparent electrical resistance R′of the resistive heater according to the fourth example of the presentinvention is equal to the electrical resistance of a circuit includingfive resistors, connected to each other in parallel, having anelectrical resistance equal to R1, R2, R3, R4, or R5. Therefore, theapparent electrical resistance R′ of the resistive heater is greatlyless than the electrical resistance estimated from the resistivity ofthe resistor 30C.

As described above, the resistive heater of the fourth example has thesame advantage as that of the resistive heater of the first example.

Above equation (2) holds for the apparent electrical resistance R′ ofthe resistive heater observed from an external driving circuit, thenumber n (n≠1) of the effective regions of the resistive heater, and thepercentage m (m<1) of each effective region in the wire resistor. In theresistive heater of the fourth example, since the branch 13C of thepositive electrode 10C and the branch 25C of the negative electrode 20Care each connected to one of both ends of the resistor 30C, the whole ofthe resistor 30C can be effectively used. Therefore, the followingequation holds:m×n=1  (3)

The following equation (4) can be obtained by substituting equation (3)into equation (2):R′=R/(n ²)  (4)

That is, the apparent electrical resistance R′ of the resistive heatercan be determined using only the number n of the effective regions ofthe resistive heater. This means that the apparent electrical resistanceR′ can be designed based only on the number n of the effective regions;hence, there is an advantage in that the resistive heater of the fourthexample can be readily designed in addition to the advantages describedin the third example.

A method for manufacturing the resistive heater (see FIGS. 4A and 4B)according to the fourth example of the present invention will now bedescribed.

FIGS. 5A to 5E are sectional views showing principal parts of theresistive heater in the order of manufacturing steps. The resistor 30Cis made of a material, such as TiN, having a resistivity of 200 μΩ·cm.TiN as well as TaN is very chemically stable as disclosed in JapaneseUnexamined Patent Application Publication Nos. 2000-294738 and 6-34925;hence, TiN is useful in achieving high long-term reliability.

First of all, as shown in FIG. 5A, the following layers are formed on aninsulating substrate 50 in this order by a sputtering process: a TiNlayer 51 having a thickness of, for example, 200 nm; an aluminum (Al)layer 52 having a thickness of, for example, 200 nm; a titanium (Ti)layer 53 having a thickness of, for example, 100 nm; and a gold (Au)layer 54 having a thickness of, for example, 500 nm. Examples of theinsulating substrate 50 include a glass substrate, ceramic substrate,and silicon substrate having a silica layer thereon. The followingprocess can be used instead of the sputtering process: a reactivesputtering process, an electron beam vapor deposition process, aresistance-heating vapor deposition process, or another process. Theresistor 30C is prepared by processing the TiN layer 51 and may be madeof TaN or another material. The aluminum layer 52, the titanium layer53, and the gold layer 54 form a conductive layer having a triple layerstructure. The conductive layer is patterned into the positive electrode10C and the negative electrode 20C. The conductive layer may have atriple layer structure consisting of a copper (Cu) layer, a chromium(Cr) layer, and a platinum (Pt) layer without including the aluminumlayer 52 and the titanium layer 53 and another type of conductive layermay be used.

As shown in FIG. 5B, a photoresist layer 60 is formed on the Au layer 54and then patterned by a photolithographic process. The resultingphotoresist layer 60 is used as a mask when the Al layer 52, the Tilayer 53, and the Au layer 54 are etched into the positive electrode 10Cand the negative electrode 20C.

The Al layer 52, the Ti layer 53, and the Au layer 54 are etched usingthe photoresist layer 60 as a mask, whereby the positive electrode 10Cand negative electrode 20C each including portions of these three layersare formed as shown in FIG. 5C. In this etching step, a wet etchingprocess or a dry etching process such as a milling process or a reactiveion etching process may be used.

The photoresist layer 60 is removed and the surfaces of the positiveelectrode 10C, the negative electrode 20C, and the TiN layer 51 are thencleaned. As shown in FIG. 5D, another photoresist layer 61 is formed onthe TiN layer 51 and then patterned by a photolithographic process. Thisphotoresist layer 61 is used as a mask when the TiN layer 51 is etchedso as to have a wire shape identical to the shape of the resistor 30C.

The TiN layer 51 is etched using the photoresist layer 61 as a mask,whereby the resistor 30C made of TiN layer 51 as shown in FIG. 5E. Inthis etching step, a wet etching process or a dry etching process suchas a milling process or a reactive ion etching process may be used.According to this procedure, the resistive heater, shown in FIGS. 4A and4B, according to the fourth example is completed.

The resistor 30C made of TiN layer 51 has a width of 10 μm, a length of2 mm, and a thickness of 200 nm. If the known resistive heater shown inFIGS. 7A and 7B includes the resistor 30C, this known resistive heaterhas an electrical resistance of 2 kΩ because the resistivity of TiN is200 μΩ·cm. In order to allow this known resistive heater to generate 300mW of heat, a power supply with a voltage of 17 V or more must be used.However, the voltage of a power supply for electronic circuits is about3 to 12 V; hence, it is useless to connect this known resistive heaterand an electronic circuit to a common power supply. Therefore, a powersupply devoted to this known resistive heater must be designed.

On the other hand, the resistive heater, shown in FIGS. 4A and 4B,according to the fourth example includes the resistor 30C made of TiNlayer 51 and the number of the effective regions of the resistive heateris equal to five, that is, n=5. Therefore, the apparent electricalresistance R′ can be determined using equation (4) as follows:R′=2 kΩ/(5²)=80ΩIn order to allow the resistive heater to generate 300 mW of heat, thevoltage of a necessary power supply is 4.9 V. Therefore, it is useful tocommonly connect the resistive heater and an electronic circuit to thispower supply.

If the number of effective regions of a resistive heater is equal toeight, that is, n=8, the apparent electrical resistance R′ thereof canbe determined as follows:R′=2 kΩ/(8²)=31.25ΩIn order to allow this resistive heater to generate 300 mW of heat, thevoltage of a necessary power supply is about 3.1 V. Therefore, it ismore useful to commonly connect this resistive heater and the electroniccircuit to this power supply.

A material for forming the positive electrode 10C and the negativeelectrode 20C is will now be described. Such a material preferablycontains at least two selected from the group consisting of gold,platinum, chromium, titanium, copper, aluminum, titanium nitride, andtantalum nitride. Other conductive elements or compounds other thanthese elements and compounds may be used.

For the resistive heater of the present invention, the resistivity ofthe positive, electrode 10C and that of the negative electrode 20C arecritical. Suppose that an imaginary resistive heater has the sameconfiguration as that of the known resistive heater shown in FIGS. 7Aand 7B and includes a positive electrode 110, a negative electrode 120,and a resistor 130 and a material for forming this positive electrode110 and this negative electrode 120 has a resistivity equal to that of amaterial for forming this resistor 130. In general, it is not rare thata positive electrode and a negative electrode have a length greater thanor equal to that of this resistor 130. If this positive electrode 110and this negative electrode 120 have a length equal to that of thisresistor 130, this positive electrode 110 and this negative electrode120 consume half of the electricity input to the imaginary resistiveheater. That is, in order to allow this resistor 130 to generate 300 mWof heat, 600 mW of electricity must be input to the imaginary resistiveheater.

Furthermore, suppose that another imaginary resistive heater has thesame configuration as that of the resistive heater, shown in FIGS. 4Aand 4B, according to the fourth example and includes a positiveelectrode 10C, a negative electrode 20C, and a resistor 30C. Since thesepositive and negative electrodes 10C and 20C have a length greater thanthat of those positive and negative electrodes included in the knownresistive heater shown in FIGS. 7A and 7B, this resistor 30C has anapparent electrical resistance R′ less than that of that resistorincluded in the known resistive heater; however, the sum of theelectrical resistances of these positive and negative electrodes 10C and20C is greater than the sum of the electrical resistances of thosepositive and negative electrodes of the known resistive heater.Therefore, there is a problem in that the amount of heat generated fromthis resistor 30C is greater than the sum of the amount of heatgenerated from this positive electrode 10C and that from this negativeelectrode 20C. Hence, these positive and negative electrodes 10C and 20Cmust have an electrical resistance sufficiently less than that of thisresistor 30C.

In the positive and negative electrodes 10C and 20C, described in thefourth example, having the triple layer structure consisting of the Allayer 52, the Ti layer 53, and the Au layer 54, each section between theconnection 11C (bonding pad) of the positive electrode 10C and the nodeP31, P33, or P35 thereof has an electrical resistance of about 1 to 3Ωand each section between the connection 21C (bonding pad) of thenegative electrode 20C and the node P32, P34, or P36 thereof has anelectrical resistance of about 1 to 3Ω. Since the number n of theeffective regions of the resistor 30C is five (see FIGS. 4A and 4B), thepositive and negative electrodes 10C and 20C are allowed to have anelectrical resistance less than 4% of that of the resistor 30C. If thenumber n of the effective regions of the resistor 30C is eight, thepositive and negative electrodes 10C and 20C are allowed to have anelectrical resistance less than 10% of that of the resistor 30C.

FIFTH EXAMPLE

FIG. 6 is a plan view showing a configuration of a thermooptic phaseshifter according to a fifth example of the present invention. Thethermooptic phase shifter includes a resistive heater, which hassubstantially the same configuration as that of the resistive heater ofthe fourth example.

The thermooptic phase shifter further includes an insulating substrate(not shown) and a straight optical waveguide extending along theinsulating substrate. The optical waveguide has a core 70, which issimply shown in FIG. 6. The core 70 is surrounded by a clad layer, whichis not shown.

The resistive heater included in the thermooptic phase shifter includesa wire resistor 30D having a predetermined length; a positive electrode10D, placed on a side (the upper side in FIG. 6) of the resistor 30D,extending along the resistor 30D; and a negative electrode 20D, placedon the side (the lower side in FIG. 6) opposite to the positiveelectrode 10D, extending along the resistor 30D.

The resistor 30D has the same configuration as that of the resistor 30C,shown in FIGS. 4A and 4B, according to the fourth example. The resistor30D is placed above the clad layer surrounding the optical waveguidecore 70 and extends in parallel to the optical waveguide core 70.

The positive electrode 10D includes a connection 11D, an L-shapedextension 12D, and three straight branches 13D, 14D, and 15D. The branch13D is connected to a node P41 placed at one end of the resistor 30D.The branch 14D is connected to a node P43 placed on the resistor 30D.The branch 15D is connected to a node P45 placed on the resistor 30D.

The negative electrode 20D includes a connection 21D, a straightextension 22D, and three straight branches 23D, 24D, and 25D. The branch23D is connected to a node P42 placed on the resistor 30D. The branch24D is connected to a node P44 placed on the resistor 30D. The branch25D is connected to a node P46 placed at the other end of the resistor30D.

An effective region 31D is present between the nodes P1 and P42 of theresistor 30C, an effective region 32D is present between the nodes P42and P43, an effective region 33D is present between the nodes P43 andP44, an effective region 34D is present between the nodes P44 and P45,and an effective region 35D is present between the nodes P45 and P46.The nodes P41 to P46 are arranged such that the five effective regions31D, 32D, 33D, 34D, and 35D have the same length. Therefore, theeffective regions 31D, 32D, 33D, 34D, and 35D have the same electricalresistance.

If the electrical resistances of the positive and negative electrodes10C and 20C are negligible, an equivalent circuit of the resistor 30C isas shown in FIG. 4B, wherein R1, R2, R3, R4, and R5 (R1=R2=R3=R4=R5)represent the electrical resistance of the effective regions 31A, 32A,33A, 34A, and 35A, respectively. The apparent electrical resistance R′of the resistive heater according to the fifth example of the presentinvention is equal to the electrical resistance of a circuit includingfive resistors, connected to each other in parallel, having anelectrical resistance equal to R1, R2, R3, R4, or R5.

The thermooptic phase shifter according to the fifth example of thepresent invention can vary the phase of light propagated through theoptical waveguide in such a manner that the resistor 30D is allowed togenerate heat by applying a current to the resistive heater and therefractive index of the optical waveguide core 70 is varied by heatingthe optical waveguide core 70 using the heat.

In order to minimize the amount of electricity consumed by the resistor30D, the optical waveguide core 70 must be efficiently heated. Since theheat generated from the resistor 30D is transmitted to the opticalwaveguide core 70 through the clad layer made of glass, the distancebetween the optical waveguide core 70 and the resistor 30D forgenerating heat is preferably small as long as optical properties of thecore 70 are not deteriorated. In this example, the resistor 30D isplaced close to the optical waveguide core 70 and extends in parallel tothe optical waveguide core 70; hence, the distance therebetween isminimum and the heat generated from the resistor 30D can therefore beefficiently transmitted to the core 70. Furthermore, the temperature ofa section, extending in the direction that light travels, for heatingthe core 70 is uniform; hence, optical properties of the core 70 can beprevented from being deteriorated due to thermal stress.

MODIFICATION

The first to fifth examples described above are intended to illustratethe present invention. Therefore, the present invention is not limitedthese examples and various modifications may be made within the scope ofthe present invention. For example, the number, position, and shape ofconnections, extensions, and branches of positive and negativeelectrodes may be arbitrarily varied as required.

As described above in detail, according to the present invention,although a resistor is made of a material, such as tantalum nitride ortitanium nitride, having a relatively large resistivity, the apparentelectrical resistance (the superficial electrical resistance of theresistive heater) is less than the electrical resistance estimated fromthe material. Therefore, the amount of heat generated from the resistiveheater can be controlled with a simple electronic circuit. The apparentelectrical resistance of the resistive heater can be adjusted to anyvalue. Accordingly, the present invention is exceedingly useful inmanufacturing a wire-shaped resistive heater controllable with a simpleelectronic circuit.

1. A thermooptic phase shifter comprising: an optical waveguide; and aresistive heater comprising: a wire resistor; a first electrode, placedon a side of the resistor, extending along the resistor; and a secondelectrode, placed on the side opposite to the first electrode, extendingalong the resistor, wherein the first electrode is connected to aplurality of first nodes placed on the resistor with branches spacedalong the resistor, the second electrode is connected to a plurality ofsecond nodes placed on the resistor with branches spaced along theresistor, the second nodes are spaced from the first nodes in thelongitudinal direction of the resistor, and the resistor has effectiveregions each sandwiched between one of the first nodes and one of thesecond nodes that is adjacent to the first connection, for heating theoptical waveguide, wherein the resistor included in the resistive heaterextends along the optical waveguide.
 2. A thermooptic phase shifteraccording to claim 1, wherein said resistive heater further comprisingthe first, wherein the first and second nodes are alternately arrangedin the longitudinal direction of the resistor.